1. Technical Field
The present invention relates to an image display device, and is particularly suitable for an image display device using light (coherent light) with high coherency.
2. Related Art
In recent years, rapid popularization of projectors has been presented. The most significant advantage of the projection type image display device is to provide a product with an equivalent screen size at a lower price compared to a direct-view type display such as a liquid crystal television or a plasma display. However, price-reduction has recently been in progress also in the direct-view type products, and further improved performance in image quality is required for the projection type image display devices. In a projector, a light modulation element such as a liquid crystal light valve is irradiated by the light emitted from a light source, and the projection light modulated by the light modulation element is enlargedly projected on a screen, thereby displaying an image. On this occasion, not only that the image is displayed on the screen, but also that the observer is to watch the dazzle on the entire screen. The dazzle is caused by the brightness variation associated with the interference of the light beams, and is called scintillation, speckle noise, or the like.
Here, the principle of generation of the scintillation will briefly be explained.
As shown in FIGS. 14A and 14B, the light emitted from a light source 70 is transmitted through a liquid crystal light valve (not shown) and then projected on a screen 74. The projection light projected on the screen 74 is diffracted by a number of scattering members 72 included in the screen 74, and is diffused by the scattering members 72 behaving like a secondary wave source. As shown in FIG. 14B, the two spherical waves caused by the secondary wave source causing reinforcement and deadening of the light in accordance with the mutual phase relationship, thus bright and dark fringes (interference pattern) appear between the screen 74 and the observer. When the eyes of the observer come into focus on the image field S generated by the interference pattern, the observer recognizes the interference pattern as the scintillation. The scintillation gives uncomfortable feeling to the observer trying to watch the image formed on the surface of the screen as if a curtain is stretched in front of the screen.
Incidentally, in the recent projectors, development of a new light source replacing the conventional high-pressure mercury lamp has been in progress, and in particular, the expectation for the laser source as a next-generation light source for projectors has been raised from viewpoints of energy efficiency, color reproducibility, long life, quick lighting, and so on. However, the projection light on the screen by the laser source becomes the light (coherent light) with very high coherency because the phases of the light beams in the adjacent areas are aligned with each other. Since the coherent length of the laser beam extends to several tens meters in some cases, if the light beam from a single light source is divided and then recombined, the light beam combined through the light paths having a shorter difference than the coherent length causes strong interference. Therefore, there appears the scintillation (the interference pattern) clearer than in the case of using the high-pressure mercury lamp. In view of the above, reduction of the scintillation is an essential technology particularly in manufacturing the projector using the laser source.
As such measures for reducing the scintillation, the following technologies have been proposed.
The technology described in JP-A-2006-221069 (Document 1) is for varying the diffusion characteristic with time, and the screen provided with a light diffusion sheet having a polymer-dispersed liquid crystal layer and electrodes for driving the polymer-dispersed liquid crystal layer is described therein. According to this screen, by varying the diffusion characteristic of the light diffusion sheet at high rate it becomes possible to time-overlap the speckle patterns to be homogenized so as not to be conspicuous. Further, JP-A-2005-338520 (Document 2) discloses the image display device using the semiconductor laser diodes as the blue light source and the red light source, and a light emitting diode (LED) for emitting the light (incoherent light) with low coherency as the light source of green to which the human eye has a high luminosity factor.
In other words, as the measures for reducing the scintillation described above, the technology of the Document 1 is for taking measures thereto by making the scintillation inconspicuous on the screen side, and the technology of the Document 2 is for taking measures thereto by reducing the coherency of the light on the light source side, and a certain level of reduction effect can be expected. However, in the technology of the Document 1, since there is a limitation in the response rate of the polymer-dispersed liquid crystal, it is difficult to completely eliminate the scintillation. Further, regarding what takes measures on the screen side, there is a problem that the image becomes blurred with the excessively enhanced diffusivity of the light in the screen, thus making it difficult to obtain a clear image. On the other hand, in the technology of the Document 2, the incoherent light source such as an LED is required to be used, and accordingly the technology is not applicable to the case of configuring all of the light sources with the coherent light sources such as the laser sources, and therefore insufficient as the measure therefor.
It should be noted that the problem described above is prominent in the projectors using the laser source, but is not the problem specific to the projectors using the laser source. The problem is common to the image display devices including, for example, a projector using the lamp such as the high-pressure mercury lamp, and further a direct-view type liquid crystal display besides the projector.